This invention relates to metal detection equipment and particularly to the arrangement of inductor coils in a search head for a metal detector.
Commonly known electronic metal detectors have a unitary search head having one or more inductor coils and means for providing a time-varying magnetic field. Detection of a conductive or magnetic object generally depends on a change in the voltages induced in a receive coil by a transmit coil of the search head, the change in voltage being caused by a change in the mutual induction between the transmit and receive coils when an object enters the field of the transmit coil.
Metal detectors typically include a signal generator which supplies a sinusoidally time-varying voltage to a transmit coil to produce a time-varying electromagnetic field which typically may be unmodulated, amplitude-modulated, or pulsed. Usually a receive coil, which may include two portions of opposing polarity located in a known spatial relationship to the position of the transmit coil, is connected to a detector circuit which amplifies and evaluates the voltage, if any, induced within the receive coil by the transmit coil. The presence of any conductive or magnetic material within the changing flux field of the transmit coil alters the basic relationship between the transmit coil and the receive coil, causing a detectable change in the voltage induced in the receive coil.
it is common in the search head of such metal detectors for a receive coil or coils to be located overlapping a larger annular transmit coil, in a spatial relationship with the larger coil such that the receive coils are in null positions relative to the fluctuating magnetic field of the transmit coil in air. The presence of nearby metallic or magnetic objects causes anomalies in the shape of the flux field of the transmit coil, thus causing voltages to be induced in the receive coil or coils.
In more sophisticated electronic metal detectors the phase relationship between the transmitted signal and the voltage induced in the receive coil may be determined and used to help identify the object which has altered the normal electromagnetic flux pattern. When a conductive metallic object is within the field, the changing magnetic field of the transmit coil generates, within the object, eddy currents whose characteristics depend on the size, shape and material of the object. These eddy currents are out of phase with the current in the transmit coil, and the eddy currents themselves produce magnetic fields which are detectable as anomalies in the net magnetic field in the vicinity of the metal detector search head.
Non-conductive magnetic materials in the vicinity of the search head also cause anomalies in the magnetic field which produce detectable changes in the current in the receive coil, because of the hysteresis of thier induced magnetism produced by the time lag for rotation of magnetic dipoles of the material in response to the transmit coil's changing field. The phase relationship between the transmitted signal and the detectable current induced in the receive coil resulting from these phenomena characterize the object which has caused an anomaly in the magnetic flux pattern.
A problem in previously known electronic metal detectors is that in order to increase the sensitivity of a metal detector to the presence of desirable target objects, such as coils, rings, watches, and jewelry, it has been necessary to increase the strength and the size of the magnetic field of the transmit coil. Since the strength of the field established by a particular coil decreases rapidly with increasing distance from the coil, a field which can be used to detect distant objects is considerably stronger near the transmit coil than is necessary to detect nearby objects. As a result, in order to detect the very weak responses induced in the receive coil by distant targets, it has been necessary to use associated receiver and detection circuitry which has a very wide dynamic range in order to avoid overloading of the detection circuitry by the response of nearby objects. While numerous arrangements of transmit and receive coils have been disclosed by the prior art, all of those configurations are subject to the limitation that the sensitivity required to detect distant targets results in excessive sensitivity to nearby targets.
An obstacle often encountered while using electronic metal detectors to detect desirable targets such as coins, rings, and the like is that unwanted magnetic materials occur in an area being searched, either as native mineral deposits or as the remnants of completely corroded iron objects which are normally of no interest. By the use of an electronic phase analysis technique the receiver circuitry of a metal detector may be made non-responsive to the distributed non-conductive magnetic particles in the ground while maintaining a high sensitivity to conductive targets, providing an ability to ignore the response of the receive coil to the presence of such distributed magnetic material.
Another problem in the use of previously known metal detector search coil arrangements in conjunction with such phase analyzing circuitry is that, for targets very close to the surface over which the detector search head is moved, as the conductors of a transmit coil pass over a metallic target or a magnetic mineral particle the induced response signal in the receive coil may undergo a change in its phase relationship to the signal applied to the transmit coil because the vertical component of the magnetic field inside the transmit coil is opposite to the vertical component outside the coil.
In a search head having multiple transmit coils of different polarities such phase changes may be greater or may be separately associated with each transmit coil of the search head. In electronic metal detectors which include circuitry designed to identify response-producing objects by evaluation of the phase relationship between the transmitted signal and a received response, such a changing response to shallow or close-by objects may confuse the target identification circuitry. With such confusion an undesirable object detected by such a search head may cause an indication of the presence of a desirable object, resulting in search time wasted in locating a worthless object.
Known metal detector search head coil arrangements designed to provide improved sensitivity to the presence of desired target objects include McDaniel U.S. Pat. No. 3,882,374, Wheeler U.S. Pat. No. 2,451,596, and Penland U.S. Pat. No. 3,471,773. The McDaniel patent discloses an inductor arrangement in which three coplanar, concentric coils form a balanced inductor system. A transmit coil is located concentrically within one receive coil and a second receive coil is concentrically located within the transmit coil. The two receive coils are connected in series opposition and balanced by varying the size of the transmit coil, and by adjusting the number of turns in the inner receive coil, so that there is an inductive linkage between the transmit coil and each one of the receive coils alone, but a net decoupling of the transmit coil from the combined opposing receiving coils occurs in the absence of magnetic or conductive materials. As the search head approaches such materials, however, the receive coil response increases very rapidly, and thus an associated detection circuit may be overloaded by the receive coil response to magnetic minerals, preventing detection of desirable objects.
Wheeler U.S. Pat. No. 2,451,596 discloses a coaxial, coplanar arrangement of three coils in which a first coil has the largest diameter, and a second coil, connected in series opposition to the first coil, is the smallest of the three, these first and second coils being transmit coils. A third, receive, coil of intermediate size is located in a coaxial coplanar relationship with the first two, the sizes and numbers of turns of the three coils being chosen to produce substantially zero inductive coupling of the third coil to the first and second coils. In another coil arrangement disclosed by Wheeler the relative positions of the coils need not be coplanar so long as the third coil is located in the surface of a sphere of "zero normal component of magnetic-field intensity", or "neutral sphere", thus being somewhat decoupled from the combined fields of the first and second coils. According to Wheeler the first transmit coil will have no more turns, and generally fewer turns than the opposing and smaller second transmit coil.
The coplanar arrangement disclosed by Wheeler would produce a relatively rapid increase in strength of response to a target as the distance between the target and the search head decreases. An additional disadvantage of the Wheeler coil arrangement is that because of the separation between the opposing transmit coils of the Wheeler patent an apparent change in phase relationship between the signal applied to the transmit coil and the response to the object induced in the receive coil may occur as the search head moves over an object, which would cause difficulty in identification by evaluation of response phase angles.
Penland U.S. Pat. No. 3,471,773 discloses a search head coil arrangement in which a relatively large diameter transmit coil includes a smaller loop of opposite polarity and equal number of conductor turns which extends inwardly from the circumference of the transmit coil. A small receive coil is located slightly below the plane of the transmit coil, overlapping the small opposite polarity loop so that the receive coil is near a null position relative to the combined transmit coil loops. The inwardly protruding loop decouples the receive coil from the transmit coil and produces low receive coil output in the presence of non-metallic objects or undersirable metallic objects. While this coil arrangement provides good sensitivity to desired objects, the Penland device, like those of McDaniel and Wheeler, is subject to the problem of over-sensitivity to presence of detectable objects very close to the search head, and because of the asymmetry of the coil arrangement of Penland, target identification circuitry associated with the Penland search head may have to deal with different phase responses to the same target in different positions relative to the search head. Additionally, while the Penland coil system is balanced to a near zero receive coil current in the absence of magnetic or metallic material, it becomes unbalanced and therefore less sensitive to desirable metallic objects when operating in an environment containing unevenly distributed magnetic minerals.
Another problem with many prior art search heads such as that of Penland is that, because of asymmetical construction, variations in ambient temperature where the search head is used may cause imbalance among search head components.
What is needed, therefore, is a metal detector search head coil configuration which provides sensitivity to the presence of metallic or magnetic objects distant from the search head, despite the presence of nearby magnetic minerals, and which has sensitivity to nearby objects which is not so great that nearby objects can overload receiving and detection circuity. Also, any change in phase relationship between the transmitted signal and the received response signal as the search head passes over a target object should be small enough that characteristic phase relationships produced by certain types of targets may be used for their identification. Moreover, the search head should not be adversely affected by changes in ambient temperature.